There are two basic techniques used in digital-to-analog converters (DACs). These are the sigma-delta technique and the resistive or capacitive divider techniques. The sigma-delta technique is attractive because it achieves high resolution by precise timing instead of precisely-matched on-chip components such as resistors. In addition, the expertise needed to produce thin-film, laser-trimmed analog components is difficult to obtain; whereas, high-speed digital switching capability is commonplace in the semiconductor industry.
A basic sigma-delta DAC receives a digital signal which is summed with inverse feedback of the analog output signal (after being reconverted to a digital signal) to provide an error signal. The error signal is then processed through an integrator and a comparator to provide the analog output signal. The analog signal is also processed through an analog-to-digital converter (ADC) to provide the feedback signal.
Digital-to-analog converters incorporating sigma-delta modulators are oversampled, meaning that the digital input data is sampled at a frequency greater than the Nyquist rate. Sigma-delta modulators achieve high resolution by precise timing rather than by precisely-matched components. Sigma-delta modulators achieve better performance by shifting quantization noise out-of-band, thus allowing the user to filter out the quantization noise.
The sigma-delta modulator itself runs at a much higher rate than the input data rate. In order to convert the sampled digital data to the same frequency as that used by the modulator, it is desirable to interpolate the much-slower data presented to the modulator to reduce the levels of out-of-band images of the signal due to the sampled data. Furthermore, integrated circuit DACs can be configurable and often are required to accommodate different clock speeds and oversampling ratios (OSRs) using a minimum amount of integrated circuit area.